Many improved electric plug adaptors and electric power extension cords of the prior art as directed to such purposes as suppressing transient current surges, indicating the polarity of receptacle terminals, and providing diagnostic indications as to the integrity of grounding connections by signal means on the plug adaptor or on male plug of the power extension cord, e.g. U.S. Pat. Nos. 2,474,407; 3,924,914; 4,089,032 and 4,118,690. However, none of these patents are believed to disclose the present invention.